PHOSPHORUS FRACTIONATION AND SHORT-TERM MOBILITY IN THE SURFACE SEDIMENT OF A POLYMICTIC SHALLOWLAKE TREATED WITH A LOW DOSE OF ALUM (COURTILLE LAKE, FRANCE)
Résumé
Lake Courtille was a polymictic eutrophic lake prone to large pH variations and sediment resuspension. Short term P release (96 hr) under laboratory resuspension conditions for two sediment sample types, surface sediments which represent sediment accumulated over several years and trap sediments which represent current year sediment, was studied. The experiments were carried out in oxic conditions at different pH values. According to phosphorus fractionation, Fe-bound-P, Albound- P and Organic-bound-P comprised the largest phosphorus pool (80% of Total Phosphorus). Phosphorus release was from the Fe-P, Al-P and organic fractions. These represent 35% of the total phosphorus content at pH 10. At circumneutral pH, in oxic conditions, a lower quantity of P was released (roughly 4% of total P sediment content) than at pH 10. The organic matter mineralization occurring at the end of summer can also influence P release as illustrated by the difference in total phosphorus content between sediment samples of 2000 and of the last five years. This type of P release was observed in Lake Courtille. High soluble reactive phosphorus content was measured in the water column at the end of summer. Aluminium treatment was thus not efficient in preventing P release. Moreover, a higher alum dose was needed to reduce P content in the water column in order to decrease primary productivity and subsequent organic matter sedimentation.